It looks like this is a false alert based on your df output above. We had a somewhat similar situation recently and apparently the lmce team know of this false disk space reporting. One thing to keep your eye on though is that if you create a pxe booted MD and then delete in Web Admin Devices->Media Directors it does not get removed completely and it is possible if you do not monitor this that you could have GB's of space not getting freed up. Just check in /usr/pluto/diskless/ and make sure that the device ID for your 'deleted' MD's is not there... if it is delete the directory to recover the disk space.

Another possibility I ran into was related to logs going crazy. The current version of LMCE has pretty verbose logs (great for troubleshooting IF you know how to translate them into something useful). So if something minor goes wrong repeatedly (some autostart stuck in a loop for example) they can generate GB's of space very fast.